1. Field of the Invention
The invention is in the field of multi-zone drying systems particularly useful in controlling the final moisture content of a product leaving the final drying zone. The apparatus finds particular use in drying foods as for example animal feed such as dog food.
2. Description of the Prior Art
It is known within the prior art to utilize drying devices which are controlled by a programmable computer which receives input signals for controlling the final moisture content of the product. Single zone drying systems are shown, for example, in U.S. Pat. Nos. 3,905,123, 3,906,196 and 3,748,224. In aforementioned U.S. Pat. No. 3,905,123, for example, the moisture content of tobacco entering and leaving the dryer is measured and utilized together with a computer and a plurality of PID (proportional, integral, differential) controllers for regulating the dryer. Multi-zone drying systems are also known in the prior art as for example U.S. Pat. Nos. 3,564,724, 3,801,426, 3,815,254 and 2,768,629. The computing algorithms utilized in prior patents are designed to accomplish specific objectives with regard to the drying product as for example the moisture control curl compensation system of U.S. Pat. No. 3,564,724 or the caliper/production rate control for paper devices described in U.S. Pat. No. 3,801,426.
Within the technology of the food drying processes, particularly animal feed, drying machines have typically utilized laborious time consuming offline analysis of the moisture content of the final product which does not permit a real time adjustment in drying temperatures. Consequently, final product moisture content was apt to vary a great deal inasmuch as changes could be made only slowly and were not responsive to the present operating parameters at the time control was applied. Multi-zone drying systems typically utilize independent temperature adjustments for the different zones and depend a great deal upon the expertise and experience of the operator for obtaining a steady state operation with relatively constant final product moisture content.
Control of the final product moisture content is particularly critical in such areas as food products wherein too much moisture will cause rotting of the food as well as incorrect weight specifications when the product drys out in shipment. Drying the final product to a nominal moisture content often involves a great deal of wasted energy in the drying process with the necessity for increasing the drying time and temperature and thus decreasing plant production output and operating efficiencies.